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In a slow sand filter, the usual water filtration rate is about 100 to 200 litres per square metre per hour (≈0.1–0.2 m/h).

Difficulty: Easy

10 to 20 L/m2/hr
100 to 200 L/m2/hr
1500 to 2500 L/m2/hr
4000 to 5000 L/m2/hr
600 to 900 L/m2/hr

Correct Answer: 100 to 200 L/m2/hr

Explanation:

Introduction / Context:
Slow sand filters (SSF) achieve excellent microbiological removal at very low filtration rates thanks to biological processes within the schmutzdecke (biologically active layer).

Given Data / Assumptions:

SSF are not backwashed; cleaning is by scraping the top sand layer.
Rates are an order of magnitude lower than rapid sand filters.

Concept / Approach:
Design guidance places SSF around 0.1–0.2 m/h, which equals 100–200 L/m2/hr. This supports biologically mediated removal while maintaining effluent quality.

Step-by-Step Solution:

1) Recognize that SSF emphasize biological action; high rates would disrupt performance.2) Convert 0.1–0.2 m/h to L/m2/hr: 1 m/h = 1000 L/m2/hr; thus 0.1–0.2 m/h = 100–200 L/m2/hr.3) Choose the option matching this standard range.

Verification / Alternative check:
Historical SSF designs and WHO guidance cite similar ranges and operational practices.

Why Other Options Are Wrong:

10–20: too low; impractically small throughput.1500–2500 or 4000–5000: far too high; these are rapid filtration/pressure filter regimes.600–900: still high for SSF; risks breakthrough.

Common Pitfalls:
Confusing SSF with RSF; forgetting unit conversions; assuming higher rate is always better.

In a slow sand filter, the usual water filtration rate is about 100 to 200 litres per square metre per hour (≈0.1–0.2 m/h).

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